The present invention relates to a liquid crystal display device and a method of manufacturing the same.
Many liquid crystal display devices have been proposed as display devices of information apparatuses. Presently, liquid crystal display devices using a TN (Twisted Nematic) mode liquid crystal disclosed in Jpn. Pat. Appln. KOKAI Publication No. 47-11737 and an STN (Super Twisted Nematic) mode liquid crystal disclosed in Jpn. Pat. Appln. KOKAI Publication No. 60-107020 are extensively used. The TN mode or the STN mode takes an initial state in which the arrangement of liquid crystal molecules is twisted around 90.degree. or 260.degree., respectively, inside the device. Light entering the device exits with its polarized state being changed by the twisted structure of the liquid crystal molecules and birefringence.
When an electric field is applied to a liquid crystal cell including a liquid crystal layer having the structure of the liquid crystal molecule arrangement as above, the liquid crystal molecules are rearranged in the direction of electrolysis. Consequently, the twisted structure is untwisted and the birefringence is lost. The result is that incident light exits without changing its polarized state. When the liquid crystal cell is sandwiched between two linear polarizers, the optical properties of the liquid crystal layer change on the basis of this principle upon application of a voltage and this change is observed as a change in the intensity of light. Liquid crystal display devices of the TN and STN modes obtain the contrast of brightness in this way.
When compared to CRT (Cathode Ray Tube) displays, liquid crystal display devices using the above display method have the advantages that the consumption power is very small and thin display panels can be realized. Accordingly, liquid crystal display devices of this type are widely used in OA information apparatuses such as personal computers and word processors.
It is, however, difficult to say that liquid crystal display devices using a polarizer effectively use incident light. In effect, many displays ensure brightness by arranging a light source (backlight) behind a liquid crystal display device. Also, in liquid crystal display devices using a color filter, light passing through the device further decreases. As a result, stronger light sources become necessary. The power of a light source is comparable to the consumption power of a liquid crystal display device including a driving circuit. Therefore, liquid crystal display devices with this large consumption power are unsuitable for portable displays which are powered by batteries. That is, not only in liquid crystal color displays but also in black-and-white displays, increasing the brightness and decreasing the consumption power are antinomic in the conventional display systems. Accordingly, the development of a bright display system requiring no backlight is being earnestly desired.
In addition, fluorescent backlights are undesirable because eye fatigue occurs when a user keeps watching the display, and so reflection type bright displays are demanded. Also, when used as a projection display, this bright display system requiring no backlight contributes to miniaturization, a long life, and energy savings of the whole display system with a high light transmittance.
To meet these demands, liquid crystal display devices using no polarizer are proposed and an example is a White-Taylor type guest-host device (J. Appl. Phys. Vol. 45, pp. 4718-4723, 1974). This guest-host device uses a liquid crystal material formed by mixing a dichroic dye in liquid crystal with a chiral nematic phase, and has a structure in which these liquid crystal molecules and dichroic dye molecules are arranged almost parallel to the substrate surface. When an electric field is applied to this guest-host device, the arrangement of the liquid crystal molecules changes to change the direction of the dichroic dye molecules, changing the transmittance of light. Since the liquid crystal molecules take a twisted structure resulting from the chiral nematic phase, light absorption efficiently occurs due to the dye. In this device, therefore, high display contrast can be obtained in principle without using any polarizer.
Unfortunately, to achieve high contrast in this guest-host device, the spiral or helical pitch of the chiral nematic liquid crystal must be set on the order of the wavelength of light. When the helical pitch is shortened to this extent, a large number of declination lines are formed to degrade the display quality. At the same time, a hysteresis phenomenon occurs and this extremely lowers the speed of response to an electric field. Accordingly, the quest-host device is impractical compared to the TN mode and the STN mode.
Another example of the display systems using no polarizer is a display system called PDLC (Polymer Dispersed Liquid Crystal) disclosed in Jpn. Pat. Appln. KOKAI Publication No. 58-501631. This display system uses a material prepared by dispersing a nematic liquid crystal having positive dielectric anisotropy, in the form of grains with a diameter of several .mu.m, in a polymer matrix. Also, this PDLC uses a liquid crystal material whose refractive index to ordinary light is nearly the same as the refractive index of the polymer matrix and refractive index to extraordinary light is different from the refractive index of the polymer matrix.
In this display system in its initial state, liquid crystal molecules are twisted in the liquid crystal grains and the difference in refractive index is produced between most liquid crystal grains and the polymer matrix due to variations in the direction of arrangement of the liquid crystal grains. As a consequence, the device scatters light like frosted glass. When a sufficient voltage is applied to the device, the liquid crystal molecules in the liquid crystal grains are rearranged to make the refractive indices of the liquid crystal and the polymer matrix equal to each other with respect to vertical incident light. This eliminates refraction and reflection in the interface between the liquid crystal and the polymer matrix and thereby makes the device transparent. Note that the incident light need not be linear light.
Since the PDLC displays images by using the principle of operation as above, no polarizer is necessary and incident light can be effectively used, resulting in a bright display. However, a few tens of .mu.m are required as the thickness of the device to achieve a satisfactory display contrast, and consequently the driving voltage becomes several tens of V. Additionally, since the device is of scattering type, the device is effective as a projection display but inadequate as a direct-view display of, e.g., an OA apparatus.
Reflection displays in which the arrangement of a reflector or a liquid crystal material is improved by mixing a dichroic dye in nematic liquid crystal are proposed in Jpn. Pat. Appln. KOKAI Publication Nos. 59-178429 and 59-178428. Unfortunately, even these devices do not satisfactorily meet the demands for direct-view displays of OA apparatuses and the like.